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My google and search skills are not turning up anything about how these
things work in regards to multiple heads. I'm thinking they don't
have a compressor for each head/circuit, but I have no idea how they
achieve that? Maybe they have one motor and multiple compressing
pumps? Scroll of some sort or what? I'm simply very curious as I had
one today. (This house had three heads and two outside units.
These things were 30 years old. Sanyo. Two of them are working just
fine still. The one is low on refrigerant. A fast leak, evidently as it was
cooling fine just a week ago. But I don't care about these units--so if
anybody asks me about pressures and ambient and airflow (!), I SWEAR
I'm going to slit my wrists. )

But the circuits cannot all be flowing the same refrigerant. They must be separate.
How is that accomplished? A diagram would be good.

Thank you.

single compressor, single condenser, then it splits to the three different evaporators, each with a solenoid valve to turn flow on and off to that loop. variable speed compressor to maintain proper head pressures with any given situation.

Some of the older units (Fujitsu i think) didn't use an inverter comp, some had two comps and four circuits (two separate twin circuits) but kept refrigerant running through the idle unit. Most times it was ok but a few time ive seen where people have put one unit in a server room and the other one would sweat as it was in an unused office.